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Abstract

When given a choice between receiving $100 in 30 days and $110 in 31 days, most people wait for the larger, more delayed reward. However, when the choice is between $100 today and $110 tomorrow, many people shift their preferences to the smaller, immediate reward despite the same difference of $10 and one day. Reward properties—such as time to receipt—play a crucial role in our processing of rewards. However, reward processing does not only occur for individual decisions but also for social interactions; we must decide to either cooperate or compete with others for rewards. Humans stand out in the animal kingdom as exceptional cooperators, both in terms of the form that cooperation assumes as well as the nature of rewards attained. Regarding form, we are unique in the stability of our reciprocal interactions and in the scale of our cooperative coalitions, entailing multiple nation states in times of war. In terms of rewards, we are of course motivated, like all other animals, by the central survival payoffs such as food and water but also by abstract entities such as money, the promise of future support, and positions of power such as a king, president, or head of an academic department. To maintain these complicated interactions and evaluate the nature of reward, we must possess a number of prerequisite cognitive abilities. Here, we view this problem through the lens of evolutionary biology, asking which aspects of our cognitive machinery, and the social interactions it supports, are uniquely human and which are shared with other primates. Though we focus on lemurs, monkeys, and apes, we acknowledge that many of the processes we document are unlikely to be restricted to the primates, and in many cases, there is already comparable evidence from other mammals and birds. We begin by reviewing a suite of cognitive mechanisms that are involved in both human and nonhuman primate reward processing. Our review is particularly focused on the subset of situations with quantifiable rewards. We then describe the kinds of social interactions that are part and parcel of primate life, especially the highly social monkeys and apes. Lastly, we merge these two sections and consider how constraints on primate cognition may limit the complexity of primate social interactions.